1. Field of the Invention
This invention relates to a dynamic damper installed around a rotary shaft, such as a drive shaft of an automobile. The dynamic damper is for suppressing harmful vibrations occurred in the rotary shaft.
2. Description of the Prior Art
When a rotary shaft, such as a drive shaft and a propeller shaft of an automobile and the like, rotates, unbalanced rotations occur. As a result of the unbalanced rotations, there occurs harmful vibrations like bending vibrations and torsional vibrations. It is preferred that the harmful vibrations should not occur at all. However, dynamic dampers have been widely used to suppress the harmful vibrations. The dynamic dampers work in the following manner: The dynamic dampers adjust their intrinsic frequencies to the dominant frequencies of the excited harmful vibrations, convert the vibration energy of the rotary shaft to the vibration energy of the dynamic dampers by resonance, and absorbs the vibration energy of the rotary shaft.
A conventional dynamic damper as illustrated in FIG. 20 has been used for a drive shaft of an automobile and the like. The dynamic damper 600 comprises a fixing member 601 inserted onto and supported by a drive shaft "S", a cylinder-shaped mass member 602 disposed around the outer periphery of the fixing member 601, and an elastic member 603 disposed between the fixing member 601 and the mass member 602 and connecting the fixing member 601 and the mass member 602. The intrinsic frequency of this dynamic damper 600 is fundamentally determined by the mass of the mass member 602 and the spring constant of the elastic member 603. The elastic member 603 is subjected to loads in the compression/tensile direction with respect to the vibration of the mass member 602. As a result, the elastic member 603 supports the mass member 602 in the direction exhibiting the compression/tensile spring constant.
However, the above-mentioned dynamic damper 600 has a structure, in which the mass member 602, the fixing member 601 inserted onto and supported by the drive shaft "S" and the elastic member 603 disposed between the fixing member 601 and the mass member 602 and supporting the mass member 602 are laminated in the radial direction of the dynamic damper 600. Consequently, the outer diameter of the conventional dynamic damper 600 tends to be larger because of the laminated fixing member 601, elastic member 603 and mass member 602, and it is very hard to press-fit and install the conventional dynamic damper 600 around the drive shaft "S" because the outer periphery of the elastic member 603 is constrained by the mass member 602 made of metal.
Further, when the intrinsic frequency of the dynamic damper 600 should be set in a lower value, the spring constant of the elastic member 603 should be set in a smaller value, or the mass of the mass member 602 should be greater. The shape of the elastic member 603 should be longer in the vibration direction in order to set the spring constant thereof in a smaller value. This means that the outer diameter of the elastic member 602 should be made much greater, and that the mass of the mass member should be made greater in order to increase the mass of the mass member 602. Thus, when the intrinsic frequency of the dynamic damper 600 should be set in a smaller value, it is inevitable that the outer diameter of the dynamic damper 600 becomes larger. However, it is preferred to down-size dynamic dampers while maintaining the performances thereof, because the dynamic dampers are additional devices and the installation space of the dynamic dampers is limited. As described above, it is hard to down-size the conventional dynamic damper 600 while maintaining the performances thereof. Especially, when the intrinsic frequency of the conventional dynamic damper 600 should be lowered, the outer diameter of the dynamic damper 600 has increased adversely.